Famous YouTube parents The Bratayleys have revealed their son Caleb died aged 13 from an undiagnosed heart defect.

In a message to their millions of online fans, parents Billy and Katie said doctors discovered that Caleb, who died suddenly in October, had been suffering from hypertrophic cardiomyopathy.

The condition, which is hereditary, affects one in 500 people and causes an abnormal thickening in the wall around the heart, making it more difficult for the muscle to pump blood, and can be fatal.

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Katie and Billy, the heads of famous Youtube reality family The Bratayleys, have said today that 13-year-old son Caleb (right, with sisters Hayley, center, and Annie) died from hypertrophic cardiomyopathy

The family have previously revealed they have a history of the illness, which causes a thickening of the heart wall, though said doctors did not pick up any sign of it during Caleb's yearly health check

While symptoms can be mild, including dizziness and shortness of breath, often sufferers show no sign of the disease at all.

According to Medline Plus: 'In many young adults, the first symptom of hypertrophic cardiomyopathy is sudden collapse and possible death.'

Shortly after his death, Caleb's parents reveal the family have a history of the disease.

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In a statement, the family said: 'The doctor confirmed today that Caleb passed away from a heart condition called Hypertrophic Cardiomyopathy that went undetected in his yearly checkups.

'Caleb didn't have any symptoms so the doctor said there was nothing we could have done differently.

'The girls' hearts were just recently checked and thankfully look the way they should right now. The sadness of losing someone you love is unimaginable.

'Thank you to everyone who has reached out, your kindness and support continues to help our family. We miss our baked potato everyday.'

HYPERTROPHIC CARDIOMYOPATHY

Hypertrophic cardiomyopathy is a hereditary condition which affects around one in 500 people and causes a thickening of the heart muscle.

While this thickening can be benign, it can also cause problems with blood getting into or leaving the heart.

Symptoms including shortness of breath, dizziness and fatigue, though most sufferers have no symptoms at all.

The condition can be fatal, causing abnormal heart rhythms to occur, called arrhythmias, or by blocking the flow of blood.

The condition is at its most serious in young adults.

Those with a family history of hypertrophic cardiomyopathy may be screened for the condition, though this is not always the case.

Caleb, who lives at in Maryland with his parents and sisters, was rushed to hospital from his home on October 1 and died shortly afterwards.

Five days later, family and friends gathered to pay their final respects at a memorial service that was live-streamed and watched by almost 50,000 of his fans online.

The emotional service began with a video montage of Caleb's childhood - including some of his best YouTube moments.

They then included a clip of him recorded four days before his death, which showed him in a hat and coattails.

They were filming a series about history after a mammoth road trip of the original 13 colonies.

His cousin Paige said she got to spend a lot of the last year with him.

She added that everywhere he went he sought out Sushi, his favorite food, but his main attribute is that he would light up the room.

'He was a joy to be around,' she said fighting back tears. 'It truly shows what a family means. A family who would do anything for each other.

The baseball coach of his first team, Josh, said one word summed up Caleb: 'Exemplary'.

He said he was an example to the rest of his teammates and took pride in playing catcher.

'He was a great teammate. Caleb will always be on my roster,' he said.

Caleb (left) who lived at home in Maryland with his parents and their two daughters, was rushed to hospital after collapsing on October 1 and died shortly afterwards

Hypertrophic cardiomyopathy affects around 1 in 500 people, though most sufferers show no symptoms - and often the first sign of the disease is when they suffer a collapse, which can be fatal

Family friend Paula revealed some quirky stories about Josh, who was best friends with her son Luke.

Then she told those in attendance: 'I promise to live my life more like him and not take myself to seriously.'

The Bratayley family, including Caleb's sisters Annie, 10, and Hayley, 7, began recording their daily lives on YouTube in 2010, usually releasing one video every week.

The family had never revealed their actual last name and they live in Arnold, Maryland.

The have over 1.7million subsrcibers on YouTube, and during the series Caleb - a history buff - met and interviewed Republican presidential hopeful Cary Fiorina, as well as travelling to Hillary Clinton's campaign headquarters in Brooklyn to speak with supporters.

He was also an avid baseball player, and had played in a game just a few days before his death for the Gray Sox. A tribute video to Caleb released has more than 3million views.